Dentists often use instruments, such as handpieces, ultrasonic scalers, or syringes, that deliver water and air into a patient's mouth during the course of a dental procedure. A pressing concern in the use of such instruments is the risk of infection created by disease-causing microorganisms (pathogens) which build up over the course of time in both the dental unit water line (DUWL) that supplies the instrument and in the tips of the instruments themselves. The DUWL typically extends from the dental instrument to a water source originating either from the municipal water supply or from bottled supplies within the dentist's office. Contamination is particularly promoted within DUWL's because the water in these lines is frequently stagnant since the water flows only when the instrument is in use. There are two principal sources of instrument contamination. The first source of contamination is the pathogens found in the water supply that attach themselves to the walls of the DUWL's. The second source of contamination is the pathogens that are sucked into the instrument and DUWL due to backflow from the patient's mouth (the point of use).
The way in which DUWL's become contaminated is well-known. Basic principles of fluid mechanics dictate that zones of stagnation form around the perimeter of a DUWL. These zones exist because the velocity of flowing water is zero at the walls of the tube. In the absence of agitation, microorganisms breed and flourish in the form of thin biofilms. Found among these organisms are pathogens such as legionella, pseudomonas, and mycobacteria. Biofilms occasionally break off from the walls of the DUWL and float downstream into a patient's mouth, greatly increasing the risk of infection. Backflow is another significant source of pathogens. Backflow occurs when some of an infected patient's fluids are sucked into the tip of the instrument, eventually contaminating the entire instrument. The risk of pathogen transmission and infection becomes especially significant when immuno-compromised patients, such as HIV-positive victims and cancer victims, are exposed to water from the DUWL.
Conventional methods of sterilization fail to prevent the breeding and growth of pathogens in DUWL's. Simple liquid flushing does not solve the problem, as the biofilms are generally unaffected by flowing liquid. Likewise, flushing with biocide or other decontaminant is ineffective as many organisms are resistant to these chemical treatments. Finally, use of purified water sources to minimize the contaminants in the entering water flow is not a viable solution because pathogens multiply rapidly once the seal on the water supply is opened.
Autoclaving is the most effective method of contaminant control. Although the instrument or instrument head usually is detachable and therefore suitable for autoclaving, the DUWL's generally are not detachable and are too long and unwieldy to be autoclaved. In addition, DUWL's usually are not designed to withstand this sort of treatment.
Several attempts at filtering water flowing via the DUWL through the instrument have proven cumbersome and economically inefficient.
In Hansen, U.S. Pat. No. 4,950,159, a disposable cartridge filter is disclosed which uses activated charcoal in the filter. This material is ill-suited for filtering the water in the DUWL because the pore size of activated charcoal is too large to effectively filter out pathogens.
In Johnston, et. al, U.S. Pat. No. 5,204,004, the filter is not placed in the handle and the DUWL must be cut in order to install a new filter. These limitations are likely to make the instrument cumbersome for a dentist to use and time-consuming to replace. Due to the position of the filter, it does not solve the backflow problem without the use of a separate check-valve or the chemical disinfection of the DUWL between patients, thus adding cost and complexity to its use.
In Dalrymple, et. al, U.S. Pat. No. 5,474,451, a series of air/water filter housings are disclosed. However, the filtering mechanisms entail a multivalent iodine resin/halogen scavenging system. The cost of this scheme precludes frequent replacement and disposal of the filter cartridges. In addition, several of the preferred water treatment embodiments require a filter manifold approximately two inches in length which makes the dental instrument unwieldy. This unwieldiness undercuts the purpose of dental instruments designed for easy manipulation within a patient's mouth. This approach also introduces chemicals into the water which is delivered to the patient's mouth.
In Wolf, et. al, U.S. Pat. No. 5,556,279, the filter is based on a chemical method of decontamination, thus making the filters more expensive and not amenable to frequent disposal. As in Dalrymple, this approach also introduces chemicals into the water which is delivered to the patient's mouth.
Finally, in Kinsel, U.S. Pat. No. 5,554,025, the filter is placed very close to the at the tip of the instrument, in close proximity to the point of use. While this is beneficial for decontamination purposes, it can make the instrument unwieldy due to the location of the filter housing. In addition, the proposed filter design is not suitable for the actual physical orientation of liquid and air tubes in a DUWL. This approach requires the entire filter and housing to be disposed of when changing filters, thus adding to cost. Also, while Kinsel's filter arrangement might be suitable for a dental syringe, it might not be suitable for a handpiece due to the greater volume of water required to cool a handpiece.
The presence and potential harm of pathogens in DUWL's and dental instruments are well-documented and the American Dental Association has called for a solution.